1. Field of the Invention
The present invention relates to a plasma processing apparatus using plasma for performing surface processing such as etching a substrate or forming thin films on a substrate.
2. Description of the Related Art
In the field of manufacturing semiconductor devices, inductively-coupled plasma processing apparatuses are adopted as etching and CVD apparatuses. In inductively coupled plasma processing apparatuses, an induction coil formed of a few turns is disposed on the outer side of a vacuum chamber and high frequency current is supplied to the induction coil, to thereby feed power to the plasma generated in the vacuum chamber and maintain the generated plasma. In order to supply sufficient current through the induction coil, a high voltage up to a few KV is generated along the coil, and a large stray capacitance exists between the induction coil and the plasma.
The high voltage and stray capacitance cause the following two problems. One problem is that the high voltage on the coil is electrostatically coupled with the plasma via the stray capacitance existing between the induction coil and the plasma, causing local damage of the dielectric window existing between the coil and the plasma. The other problem is that while the current circulates through the coil, the stray capacitance existing in the path causes the coil current to change along the path deteriorating the uniformity of the generated plasma in the circumferential direction.
The former problem can be solved by disposing a Faraday shield between the induction coil and the plasma, as disclosed in patent document 1 (specification and drawings of U.S. Pat. No. 5,534,231) and patent document 2 (specification and drawings of U.S. Pat. No. 6,756,737).
The latter problem can be solved as disclosed for example in non-patent document 1 (Mark J. Kushner et al., “A three-dimensional model for inductively coupled plasma etching reactors: Azimuthal symmetry, coil properties, and comparison to experiments” J. Appl. Phys. 80 (3), 1 Aug. 1996, p. 1337-1344) disclosing a method for suppressing the non-uniformity of the circulating coil current by disposing a coupling capacitor on the terminal end of the induction coil. However, the method introducing the coupling capacitor disclosed above had the following problems. One problem is that the coupling capacitor has an effect to suppressing the non-uniformity of the circulating current along the circumference, but cannot make perfectly the current uniform. The quantitative description thereof will be provided later. The second problem is that the optimum value of the capacitor must be varied according to operation conditions (operation plasma density, pressure, used gas conditions, etc.), but it is impossible to replace the capacitor corresponding to such various conditions in mass production apparatuses. The attempt to adopt a variable capacitor as the capacitor causes increase of cost of the control unit and complication of operation methods. The third problem is that the coupling capacitor must have a property to endure a few tens of KVA, according to which the geometrical shape of the capacitor is increased, and the attempt to mount the capacitor in a matching box disposed on the upper portion of the apparatus causes the matching box size to increase, leading to problems related to mounting design.
The second and third problems are especially significant in apparatuses having an induction coil group divided into two systems, an inner circumference coil and an outer circumference coil, and enabling the coil current ratio of the inner and outer coils, to be varied so as to control the radial uniformity of the plasma according to operation conditions (refer to patent document 2 and patent document 3 (specification and drawings of U.S. Pat. No. 5,777,289), for example) and in apparatuses capable of having the voltage applied to the Faraday shield controlled to adjust the degree of capacitive coupling with the plasma and to thereby stabilize the inner wall status within the vacuum chamber (refer to patent document 2 and patent document 4 (specification and drawings of U.S. Pat. No. 5,817,534), for example).